index fossiln. The fossil remains of an organism that lived in a particular geologic age, used to identify or date the rock or rock layer in which it is found. Also called guide fossil.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.index fossil n (Geological Science) a fossil species that characterizes and is used to delimit a geological zone. Also called: zone fossil Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014in?dex fos`sil n. a widely distributed fossil, of narrow range in time, regarded as characteristic of a given geological formation and used esp. in determining the age of related formations. [1895?1900] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.index fossil – One useful for dating and demonstrating the relation between strata in which it is found.See also related terms for useful.Farlex Trivia Dictionary. © 2012 Farlex, Inc. All rights reserved.